1. Field of the Invention
This invention is directed to systems, devices and methods for converting ammonium salts, especially ammonium phosphate and ammonium sulfate and mixtures thereof, into bioorganic-augmented high nitrogen-containing inorganic fertilizer. The invention is also directed to products produced by processes of the invention.
2. Description of the Background
The disposal of sludges discharged from large-scale wastewater treatment plants is a serious and growing problem. In 1990, the United States Environmental Protection Agency indicated that a family of four discharged 300 to 400 gallons of wastewater per day. From this wastewater, publicly owned treatment works generated approximately 7.7 million dry metric tons of sludge annually or about 64 dry pounds of sludge for every individual in the United States. By the year 2000, these figures had doubled.
The definitions of “sewage sludge” and “sludge” under by Title 40 of the Code of Federal Regulations, Part 257.2, hereby incorporated by reference, is as follows:                “Sewage sludge means solid, semi-solid, or liquid residue generated during the treatment of domestic sewage in a treatment works. Sewage sludge includes, but is not limited to, domestic septage; scum or solid removed in primary, secondary or advanced wastewater treatment processes; and a material derived from sewage sludge. Sewage sludge does not include ash generated during the firing of sewage sludge in a sewage sludge incinerator or grit and screenings generated during preliminary treatment of domestic sewage in a treatment works. Sludge means solid, semi-solid or liquid waste generated from municipal, commercial, or industrial wastewater treatment plant, water supply treatment plant, or air pollution control facility or any other such waste having similar characteristics and effect.”        
There are several types of sludges that can be produced by sewage and/or wastewater treatment. These include primary sludge, waste activated sludge, pasteurized sludge, heat-treated sludge, and aerobically or anaerobically digested sludge, and combinations of all. These sludges may be from municipal and/or industrial sources.
Most commonly, sludges are dewatered to the best extent possible by chemical and mechanical means. The water content of sewage sludges is still very high. Typical sludges coming out of a gravity clarifier may have a dry solids content of 2% or less. After anaerobic digestion, the solids content can be about 10%. Cationic water-soluble polymers have been found useful for causing further separation between the solids and the water that is chemically and physically bound. Filtration or centrifugation of cationic polymer treated sludge typically yields a paste-like sludge cake containing about 20% solids.
Drying of sewage sludge has been practiced for many years in both the United States and Europe. Sludge drying in the United States prior to about 1965 was undertaken to reduce transportation costs and in pursuit of various disposal options. In some plants, the sludge was dried in powder form and the fine particles were consumed in the combustion chamber of an incinerator or boiler. In the late 1960's two municipalities, Houston and Milwaukee, began to market a pelletized or granulated dried sludge for use as a soil amendment and/or fertilizer. Several more plants for manufacture of dried pelletized sludge were built in the 1980's and 1990's; especially after ocean dumping of sludge by coastal cities was eliminated. Drying and conversion to a pelletized fertilizer was the best option for these metropolitan areas where landfills and land for disposal were limited. However, the investment required for a sludge drying facility is large. A typical unit costs about $150 million for equipment alone.
The most common type of sludge dried and pelletized is anaerobically digested municipal sewage. Anaerobic digestions, as the name suggests, involves treatment by facultative bacteria under anaerobic conditions to decompose the organic matter in the sludge. After a prescribed time and temperature, a sludge relatively free of putrifiable organic matter and pathogens is obtained. Municipal anaerobically digested sewage sludge is therefore preferred for agricultural purposes.
However, dry sewage sludge has several disadvantages for agricultural use. It has low fertilization value, typically having nitrogen content of only about 2-5%. Freight and application costs per unit of nitrogen are high. It often has a disagreeable odor, particularly when moist. It has low density and when blended with other commercial fertilizer materials, it may segregate into piles or may not spread on the field uniformly with other more dense ingredients. Bacterial action may continue and under storage conditions sludge temperature may rise to the point of autoignition. Hence, except for special markets that value its organic content for soil amendment or filler in blended fertilizer, there is little demand for the product. In most cases municipalities must pay freight charges, or may offer other incentives for commercial growers to use the material. However, this is frequently still more economical than alternative disposal schemes.
The market value for fertilizers is principally based on their nitrogen content. A need exists for a practical and economic method for increasing the nitrogen content of sewage sludge to a level approaching that of commercial mineral fertilizers, i.e., 10-20%. Freight costs and the cost of application per unit of nitrogen would then be much lower. Overall value and demand would increase. Moreover, sludge has an advantage in that its nitrogen is of the slow release type. The nitrogen is part of organic molecules and hence is available to growing plants only when the molecule is broken down. This is very desirable since it provides nitrogen to the plant all through its growing cycle. Manufactured slow release nitrogen fertilizers have a price nearly 10 times that of ordinary mineral nitrogen fertilizers. Conceivably, municipalities would enjoy a credit rather than an expense in disposing of their dried sludge product if the total nitrogen content can be increased and the tendency for autoignition reduced or eliminated.
Prior attempts have been made to reach some of these objectives. U.S. Pat. Nos. 3,942,970, 3,655,395, 3,939,280, 4,304,588, and 4,519,831 describe processes for converting sewage sludge to fertilizer. In each of these processes a urea-formaldehyde condensation product is formed in situ with the sludge. However, the processes require the handling of formaldehyde, a highly toxic lachrymator and cancer suspect agent.
French Patent No. 2,757,504 describes the blending of mineral fertilizers with organic sludge. The mixture is heated to a temperature between 200° C. and 380° C. Japanese Patent No. 58032638 describes a process where sludge is treated with sulfuric and nitric acids or sulfuric and phosphoric acids and ammonia under elevated pressure of about 3 atmospheres. These prior art processes require costly process equipment and/or special conditions not readily incorporated in existing sewage treatment facilities.
The simplest method of increasing the nitrogen in sludge would be to add commercial nitrogen fertilizer materials to the wet sludge prior to drying and pelletizing. There are only a few high-nitrogen fertilizer materials that are economic for use in agriculture: ammonia (82 wt. % N), urea (37 wt. % N), and ammonium nitrate (35 wt. % N). Ammonia has high volatility and is subject to strict regulation of discharges to the atmosphere. Urea is a solid that adsorbs moisture quite readily and makes the sludge more difficult to dry. It is also highly susceptible to breakdown to ammonia by the microbes and enzymes in sludge, resulting in nitrogen loss and an odor problem. Ammonium nitrate is a strong oxidizer and creates a potential explosion problem. All of these fertilizers have high nitrogen content: but are unsuitable for combining with sludge.
Another possible candidate that has been unsuccessfully tested by the industry as an additive to sludge is ammonium sulfate. Although ammonium sulfate has lower nitrogen content (21 wt % N) than the materials discussed above, it has a price per unit of nitrogen comparable to that of the other commercial fertilizers. It is also relatively inert to the microbes and enzymes in sludge.
It has been found in full-scale plant trials that a problem occurs during the drying of a mixture of ammonium sulfate and sludge. Title 40 of the Code of Federal Regulations, Part 503, Appendix B specifies that the temperature of the sewage sludge particles must exceed 80° C. (176° F.) or the wet bulb temperature of the gas in contact with the sewage sludge must leave the dryer at a temperature exceeding 80° C. (176° F.). However, when drying a mixture of ammonium sulfate and sludge, a sudden release of ammonia vapors occurs at about 60° C. (140° F.) overwhelming the air pollution control system. Several attempts at addition of ammonium sulfate to sewage sludge in several different plants over several years have foundered on this problem. The discharge of ammonia to the atmosphere is environmentally intolerable. Consequently, ammonium sulfate addition to sewage sludge has not been successful to date.
European Patent No. 0143392 B1 describes a process in which an undigested liquid sludge is mixed with salts such as ammonium sulfate at a concentration of 17-47 wt % at a pH of 2-6 for a period of 3 to 12 hours followed by disposal. Japanese Patent No. 9110570 A2 describes the treatment of sewage sludge with an acidic solution followed by drying to reduce ammonia evolution and to retain the effective nitrogen. Therein is described the use of dilute (0.3 Normal) aqueous solutions of HCl, H2SO4, and wood vinegar as ammonia binders (“Granulation of Compost From Sewage Sludge. V. Reduction of Ammonia Emission From Drying Process”, Hokkaidoritsa Kogyo Shikenjo Hokoku, 287, 85-89 (1988)). None of these references disclose the use of acids with ammonium sulfate additions and neither reference discusses the issue of corrosion of steel process equipment under acid conditions.
Over the past thirty years alkaline stabilization of sludges has been a standard and successful method of making sludges into beneficially useful materials that can be used principally as soil-conditioning materials. Because these sludges have high calcium carbonate equivalencies, they have been produced and marketed as AG-lime materials, usually as a replacement for calcium carbonate in farm soil management strategies. Because of this usage the value of these materials has been restricted to only a few dollars per ton of product, they are economically and geographically restricted because of transportation costs to areas close to the source of their treatment. Many of these alkaline-stabilized sludges contain up to 65% water.
Thus, there is a long standing need for practical means of increasing the economic value of sewage sludge through increasing its nitrogen content, and increasing its ability to be spread as well as a need to treat these materials such that they are converted into commodity fertilizers with physical and chemical and nutrient properties such that they can command significant value in the national and international commodity fertilizer marketplace. The present invention meets those needs.